1. Field of the Invention
The present invention relates to a switching power-supply circuit and a semiconductor integrated circuit that generate a predetermined output voltage by controlling a first switching transistor connected to a power supply and a second switching transistor connected between the first switching transistor and the ground.
2. Description of the Related Art
As a power-supply circuit that efficiently converts an input voltage into a given output voltage, a power-supply circuit of a switching regulator type is known. Also used is a power-supply circuit having plural channels through which the power-supply circuit is capable of supplying different output voltages respectively to plural loads. In the switching regulator, further, a soft-start operation takes place for the purpose of suppressing an inrush current passing through a switching element at the time of start. As employed herein, the “soft-start operation” refers to the operation of, after the start, gradually increasing a conducting (or on-duty) period of the switching element in order to gradually raise the output voltage.
The schematic configuration of the power-supply circuit mentioned above is illustrated in FIG. 1. A semiconductor integrated circuit 100, in FIG. 1, includes switching controllers 101 and 102 mounted thereon. The switching controller 101 controls the switching of switching transistors Tr1a and Tr2a, and the switching controller 102 controls the switching of switching transistors Tr1b and Tr2b. 
The switching controllers 101 and 102 switch rapidly the switching transistors Tr1a and Tr2a, and the switching transistors Tr1b and Tr2b, respectively, to convert a power-supply voltage Vcc from a power supply into pulses. The pulses are smoothed into constant output voltages Vo1 and Vo2 by smoothing circuits 201 and 202, respectively. Detectors 311 and 312 detect the output voltages Vo1 and Vo2, respectively, and then output the detected voltages as feedback voltages Vinv1 and Vinv2, respectively.
To carry out the soft-start operation, the power-supply circuit shown in FIG. 1 also includes constant-current sources 131 and 132, and external capacitors C1a and C1b connected respectively to soft-start terminals 303a and 303b. The capacitors Cla and C1b are charged with a constant current I supplied by the constant-current sources 131 and 132, respectively, to generate soft-start voltages Vsof1 and Vsof2, respectively.
During a given period of time after the start, the switching controllers 101 and 102 perform switching control so that the feedback voltages Vinv1 and Vinv2 coincide with the soft-start voltages Vsof1 and Vsof2, respectively. As a result, the output voltages Vo1 and Vo2 can be raised gradually.
Decreasing either the soft-start voltage Vsof1 or Vsof2 to the ground voltage (i.e., 0 V) makes it possible to turn off either the output voltage Vo1 or Vo2, which will be hereinafter referred to as “independent turn-off.” For carrying out the independent turn-off, a controller 103 brings either a switch SW1 or SW2 into conduction to discharge either the capacitor C1a or C1b, in accordance with an independent turn-off control signal CTL inputted to a terminal 301. Incidentally, a terminal 300 is the terminal supplied with an input of a start signal for starting the semiconductor integrated circuit 100.
For the purpose of discharging a capacitor for soft start, in the case of restarting after the stop of the supply of an output voltage, proposed is the approach of connecting a switching element across the capacitor for soft start (e.g., Japanese Patent Application Publication No. H7-67322).
For achieving the independent turn-off, the power-supply circuit shown in FIG. 1, however, requires the terminal 301 supplied with an input of the independent turn-off control signal CTL and the switches SW1 and SW2, resulting in increases in the number of terminals and the scale of circuitry.
Connecting the switching element across the capacitor for soft start makes it possible to avoid the increases in the number of terminals and the scale of circuitry. Due to the parasitic resistance of the switching element, the soft-start voltage, however, cannot be reliably decreased to 0 V and floats on the order of 10 mV above the ground voltage (0 V). It is therefore difficult to completely turn off the output voltage.